The present invention broadly relates to the art of feed controls, particularly for welding operations, and more specifically concerns a new and improved method of, and apparatus for, controlling the start of the welding operation and the completion of the welding operation during continuous resistance welding. The resistance welding can be accomplished by means of alternating current at a roll-welding machine.
During the fabrication of barrels or other containers, for instance tin plate cans comprising a substantially cylindrical shell or jacket and a folded-in floor or bottom the heretofore known state-of-the-art processes advantageously relied upon the so-called resistance roll seam welding for realizing a leak-tight seam. Resistance roll seam welding is characterized as overlap welding of a sequence of spot welds, and during each half-wave of the alternating-current employed for this purpose there is applied a weld spot. This weld spot, considered with respect to the direction of transport or feed of the workpiece to be welded, has a predetermined length dimension which results from the speed of the transport mechanism of the roll-welding machine for a predetermined frequency of the alternating-current, and related to practical considerations, amounts to approximately 2.5 millimeters with a transport speed or velocity of 15m/min. and an alternating-current of 50 Hz. The weld spots of the aforementioned length then are located on a line next to one another along the overlapping edges of the workpiece to be welded.
With the present day conventional welding techniques it is, however, unavoidable that at the start of the weld seam and/or at the end of the weld seam a weld spot appears exactly at the workpiece edge, resulting in only a portion of such weld spot being located at the workpiece to be welded, which, in turn, causes a burning or scorching of such edge. If the workpiece to be welded is for instance a rounded blank which forms the body of a can which is to be fabricated in this manner, and which body is subsequently provided with an end unit by carrying out a seaming operation, then the drawbacks of the scorched edges become particularly perceivable during the seam of such end unit since leakage locations appear in the seam.